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Are transponder codes 7701 - 7777 ever used?
I understand that blocks of transponder codes are assigned to different area controls. I am curious if the block 7701 to 7777 is in use anywhere? Maybe the emergency code 7700 needs some extra protection?

According to AOPA, 7777 is used by military for interception.

I do not understand that. Should I set 7777 if I realise I am being intercepted?

Maybe it's used by the interceptor. Otherwise, I read different things for code 7776 and 7777, about being used to monitor SSR. Also it seems many pilots receive 77xx (except 7700) for regular use, from time to time. On the other hand 7700 is not used for each emergency, ATC is able to associate an emergency sticker to a regular code, however 7700 (and other special codes) initiated by the pilot trigger an alarm at ATC.

@Wirewrap I was trained to handle being intercepted as follows: If talking to ATC, do not change your transponder code. Notify ATC that you've been intercepted. If not talking to ATC, squalk 7700. Being intercepted meets the definition of an in-flight emergency.

Yes these codes are used. Only 7700 is used to denote emergencies, and usually only if you aren't in contact with ATC, or can't get their attention(frequency is packed). Codes in this range are allocated according to the National Beacon Assignment Plan in the US. There are other codes that are assigned for various issues, 7777 as mentioned in the comments is assigned, and I've assigned others in that range before, same with the 7600 and 7500 ranges.
If you're already in contact with ATC, and they have you tagged, it is very easy to mark you as an emergency and have it light up on everyone in the facility's scope.

Just one thing to be aware of when you are assigned a code in the 7500, 7600 or 7700 blocks; always make sure that as you're punching up the transponder code, you never have the display set to the reserved code even for a second. For instance, if you're squawking VFR (1200) and are told by the tower to squawk 7732, after you've set the first digit to seven, when setting the second digit you'll transition through 7500 (hijack), 7600 (no comms) and 7700 (emergency), lighting up the tower's systems.

To avoid this, get in the habit of setting the digits in reverse order when moving from VFR to a tower-assigned code, and (if you don't have a one-button VFR function on your transponder) setting the first digit first when moving to VFR. There can still be some gotchas, but overall you're much less likely to create a false alarm.

You should still use the emergency code even if you are in contact with ATC, because you will also light up on other facilities' screens then. Unless you are already in an airport control zone, there will be at least one handover before landing.

Newer units have pushbuttons for code entry, they don't seem to change what is being squawked until the 4th digit is pressed. See GTX345 at Garmin.com for an example.
See at 1:25 into this video where 1200 goes away and the code starts going in.
https://youtu.be/-BHp47RJbYk?t=75  See that? The nice picture went away when I changed from Answer to Comment.

Airman's Information Manual AIM 4-1-20 e.2 says, "Under no circumstances should a pilot of a civil aircraft operate the transponder on Code 7777. This code is reserved for military interceptor operations."
(AIM 2016 edition, seems to be the latest edition as of 2017 Sep)
AOPA flight training magazine Nov 2017 advises temporarily putting your transponder to standby while changing numbers so that you don't accidentally trigger alerts in towers by passing through the special 7xxx numbers.


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where are filed flight plans / routing stored? or: where does flightaware get route data?
One of my favorite new distractions is to look at the routing information that flightaware.com makes available.  If we pull this one, for example:  http://flightaware.com/live/flight/UAL98/history/20170123/0635Z/KLAX/YMML we can see the route is described as:
PRCH2 DINTY R576 DIALO R576 DENNS BARKR B474 ADOWA B474 ARTOP B474 SON BODEG 2300S/16000E HARVS SCOTT Y32 CG Q53 ISKIM Q94 CANTY H119 ARBEY
Which I can plug into ForeFlight or other flight planners or whatever.  My question is: where does flightaware get this data?  I have to assume that it's created by some combination of the airline dispatch groups and filed with the FAA, and lives in "some database" that the FAA/ATC uses.  But what mechanism does flightaware.com use to get this information?  And is there a way (preferably programmatically) that I can get access to the routing filed for a particular flight or tail number?

For an overall introduction of how flight plan data are managed in Europe: Eurocontrol Flight planning and Flight data services. This is similar for other flow management agencies.

source: Stack Exchange

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Why do some airports have a non standard traffic pattern altitude?
Some airports have a non-standard traffic pattern altitude.  For example KGXY is 800 feet.  Why is this?

Airports underneath controlled airspace may need to keep their traffic clear of controlled airspace, which may start from a typical altitude such as 1000Ft.
As is also mentioned in this answer, in April 2013 the standard pattern height was changed from 800 ft to 1000ft, possibly accounting for a number of airports still having the old "standard".

Many airports have non-standard traffic pattern altitudes (lower, in my experience) that are not due to airspace considerations. For example, KACQ has a TPA of 800 AGL, despite Class E starting at 700 above the airfield and continuing up to the floor of A.

See also KMCW, KOWA, and KAEL.

@JonathanWalters I didnt say "All airports with non-standard pattern altitudes are beneath controlled airspace" - crucially there was a "may" in there. Im sure there are many reasons, and I was speaking mainly from personal experience of small airfields beneath the London's extensive  control area.

Oh, I agree! I just think there is more to this question than airspace. I don't know what it is, but you have covered the airspace part of the answer. Airspace also seems a non-issue in the case of KGXY from the question.

I just talked to the airport manager at ACQ, and he says he has no idea, just the way it is. "Some airports have 800 foot patterns, some 1000."

@JonathanWalters, this answer mentions the default used to be 800 and then it was changed to 1000.

@JanHudec Well, then, that seems to be the other part of the answer, if not the main answer.

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What happens when autopilot and propulsion settings do not match?
Autopilots used in piston GA usually do not have throttle control. They only manage the control surfaces. However to trim an aircraft one needs to play on both throttle setting (and more physically, thrust) and control surface deflections (aerodynamic forces). What happens if the autopilot cannot trim the aircraft due to the propulsion settings? Is there any alert from the autopilot for the pilot?

It all depends on the autopilot, but generally no. If for example, you tell the autopilot to climb at 300 fpm, it will keep adjusting pitch to do just that.   As you climb higher (without changing the power) the autopilot will keep increasing pitch to maintain the set vertical speed.  Eventually the airplane will get too slow because of the increased pitch and can even stall.  Some autopilots have pitch limits or monitor the stall inputs and automatically disconnect before it gets to this point, but others don't.  It is very important to know your autopilot and to monitor it at all times to make sure that it is doing what you want.

Some autopilots also have IAS (indicated airspeed) setting which can be used for climbing. If you have this the IAS will stay constant but the vertical speed will decrease as you gain altitude.

@guidoism - Absolutely! I was not trying to explain all of the modes of an autopilot (there are others as well) but just have an example to show what can happen in this situation.


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Why should isobars be aligned with the leading edge of the main wing for a transonic cruiser?
When designing an airplane meant to cruise at transonic speeds (or supersonic speeds), I heard that one should look at the isobars on the main wing in order to assess if the shape, sweep angle and other airfoil parameters are suitably chosen. I also read that the better configuration yields isobars that are parallel to the leading edge. I already know that there are several techniques to get the isobars aligned with the leading edge (area ruling, adapting the sweep angle, extra care in the design of the junction between the fuselage and the wing), but why is this configuration better than another one? What does it improve?

I finally found several arguments to answer this question from different sources.
Sweep and Mach Number from Airbus
This argument is developed in an Airbus magazine from 1985. To grasp it it is important to understand the purpose of swept wing. By sweeping the main wings one introduces an angle between the leading edge of the wing and a line perpendicular to the fuselage axis. Let's consider two airplanes, one with swept wings (A) and one with straight wings (B).
One of the most important parameters is the Mach Number experienced by the wings. It is measured normal to the leading edge of the wing which means that for the two airplanes flying at the same Mach Number, the Mach Number seen by wings of (B) is larger than the one seen by the wings of (A). This allows to fly at higher speed because it delays the large increase in drag (drag divergence) and other effects linked to the Mach Number around the airfoil.
However, if the isobars on the wing are not aligned with the leading edge, they are following a less swept pattern which tends to reduce the benefits from the sweep angle of the main wing because the effective sweep angle is smaller and the Mach Number around the wing should not be computed normal to the leading edge but normal to the isobars close to the leading edge.
From a theoretical point of view it would even be better to have isobars not aligned with the leading edge but having a larger sweep angle. However this never happens due to boundary conditions at the root and the tip of the wing.
Other sources
All this is also explained here.
Finally, this last source deals also with the matters but does not justify the need for the isobars to be parallel to the leading edge. It deals more with the shock on the wing and the ways to weaken it and thus reduce the drag.

The aim of parallel isobars on the wing is to generate quasi two-dimensional flow.
Let's say, the layout is set. You shaped the wing in a way to 'force' the isobars to be parallel with the sweep - the entire span.
You now (should) have the same pressure distribution in every single profile of your wing. This eases your calculations extremely!

It obviously eases a lot the predictions and the computations but is there any aerodynamic advantage to use this? Does it improve the drag, the lift, some kind of moments and so on...?

I don't think that this has a direct effect on drag etc.  If someone knows better I'll retract everything and assert the contrary ;)

@ClickOKtoTerminate: The two main advantages are no crossflow and the same critical $c_p$ over the whole span. Flow direction is orthogonal to isobars, and having the same spanwise pressure over the whole span will ensure that airflow is well behaved. Also, the same pressure means that the wing has an optimum thickness distribution and every part of it will have the same maximum Mach number.

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How can I tell apart an Airbus from a Boeing?
When looking at an airplane, what are the main external features I should look at if I want to determine if it is an Airbus or a Boeing (shape of the tail, fuselage, winglets, wings, etc.)? Same questions from a passenger point of view, quietly seated inside?

I'm sure this is one of those dangerous questions such as how do you tell the difference between a Ferrari and a Corvette... [Actually a good question.]

Defining specific types where the confusion exists would make the question even better. :)

I like to train being able to determine the precise model of an aircraft.
I first try to tell if it's a Boeing or Airbus, and then I look at other details to determine the model.
The idea is to learn several differences :

cockpit windows shape (Boeing has an very specific angle) ;
nose shape (pointy nose on Boeing, "fat nose" on Airbus) ;
wingtips (winglets Boeing, fence Airbus) ;
number of engines (A340, B747, A380 have 4 engines, all others have 2) ;
gear.

Since there are already nice answers describing most of these points with pictures, I will focus on the gear to add a new item to the list.
Landing Gear
Looking at the gear adds a new hint. Some landing gears are unique, which means that only looking at them will be enough to determine the precise aircraft type. These very specific gears are :

B777
B747
A340 (2 different kinds for this aircraft only)
A380

It's not worth to look at the nose wheels, because there are always 2. But the main landing gear is helpful.
I made a gear cheat sheet to help me learning them.

Once these differences have been learned, the key is to use several of them to crosscheck and ease and speed up the recognition.
For example if I see the gear
   2
2*2 2*2

I will then check the cockpit windows to determine whether Boeing or Airbus.
If it's Airbus, I will know it's very likely to be an A330! If it's Boeing, it will be harder since there are still 3 different aircrafts. If I want to be able to distinguish between them, I can learn new specific hints for this purpose.

The "winglets Boeing, fence Airbus" statement isn't entirely true. Have a look at A320neo (Wikipedia link)

The question is vague, not specifying on which Boeing and Airbus pair the problem lies on. For example Boeing 377 Stratocruiser is also a Boeing but I think there is very small possibility for someone to confuse it to an Airbus. Having said that, I consider a random pair of comparable aircraft, A320 (the left) and B734 (the right).
Nose
B734 has a more streamlined and "pointy" nose and a more streamlined "forehead". Windshield is also different on B734 making that characteristic corner plus the "eyebrows" which are not though standard on all 737's (per Wikipedia). Also note the much shorter nose gear of the 737.

Engines
B734 engine is thinner with the characteristic flattening at the bottom of it. The engine cover is also longer in B734.

Also if you have the chance to have a front side look of the planes, then the flattening of 737's CFM is even more prominent.

Since the blueprint can't be as descriptive as a picture, you may have a look in this question where there are some excellent photos of the 737's engines
Jan Hudec correctly pointed out that Airbus A320 comes with 2 different engine types. I would guess from the blueprint that the pictured A320 "wears" the CFM56.
Tail
B734 has a dorsal fin (present from B733 onward) while A230 doesn't. Also the fin and horizontal stabilizers/stabilators are located much closer to the fuselage end in B734 compared to A320

Those are the obvious (in my opinion) differences. For further you can Google (as already mentioned), simply observe (as I already mentioned but deleted it) and do the same for any other pair of aircraft you have difficulties.
Images are from wikipedia, so I think it will be easy thing doing the same for other types as well.

The angled windows with angled lower edge that look a bit like the aircraft is frowning appears on the larger Boeing jets as well. The engines on the other hand are specific to 737, because it was originally designed with smaller engines and thus with much lower ground clearance so the engines had to be squeezed a bit. As for the cowling length, A320 has two types of engines and while CFM56 have short cowling, IAE V2500 have long cowling.

@JanHudec good observation the one with the 2 different engines of the A320. I've updated the answer. For the angled windows thing, yes they do appear in bigger planes (707 for example) but unlike the OP, I want to be specific in my answer. Besides, the comparison is Airbus vs Boeing not Boeing vs another Boeing.

Externally
The empennage (tail section) is consistent on most Boeings and Airbuses. An Airbus will have a bit of the fuselage sticking out behind the tail planes, whereas most Boeing aircraft have the tail planes right up against the rear of the fuselage.
Internally
The internal appearance of airliners vary widely since it is up to the airline, not the manufacturer.
Comparison
Airbus a300

Boeing 737

fuselage cross section is also a dead giveaway. Airbusses have an almost perfectly circular cross section, Boeings are more oblong.

If you're actually sitting in one, the evacuation instructions card in the seatback pocket should say what model it is.

Further to the above answers, at night Airbus aircraft can be identified due to their unique strobe flash sequences. While all other aircraft strobes flash once, Airbus strobes flash twice, i.e
flash flash pause flash flash pause -  Airbus
flash pause flash pause - Non-Airbus aircraft

It would be great if you could add a reference or source.

According to a thread here, http://www.airliners.net/aviation-forums/tech_ops/read.main/142392/ , "Airbus uses a direction identification logic in their strobe lights. Forward facing strobes (left and right wingtip) double flash, whereas the rearward facing strobe is a single flashing unit. As a result you can identify whether the aircraft is coming towards you or away from you by the strobe flashes (double = towards you, single = away from you)" and "Airbus does have a synchronization between the strobe and the beacon lights. heres an example if what it looks like

1=1st wing strobe
2=2nd wing strobe
3=beacon
1.2..3..1.2..3..1.2..3..1.2..3 so on and so forth"

Externally, focus on the fuselage shape.  The top of the fuselage on an airbus runs level until very near the aft end.  A Boeing fuselage is more conical at the rear end, with the top curving down to meet the bottom line curving up.
Internally, look at the safety briefing card.  Remember, the interior fit out of planes is determined by the airline, not the manufacturer.

I'd add that the shape of the nose can help when viewed externally (although I noticed newer Boeing and Airbus planes have greater variance here, making this less helpful).

Also, internally, Airbus uses sidesticks whereas boeing uses yokes. But then again, if you're up front, you're hopefully already aware of what kind of aircraft you're flying! :)

@JeffBridgman - This is not a catch-all, but: Boeings (especially the older models) tended to have cockpit windows that were sloped differently than rest of the nose, kind of like a car windshield. This has gone from extreme (the 247) to almost nothing (the 787). They are also usually quite high above the tip of the nose. Airbus on the other hand has almost always had their windshields flush with the rest of the aircraft, and they tend to be closer to the tip.

When looking at the front of the plane, I usually look for the clipped outer-upper cockpit window corners. The Boeings have a 90-degree corner, the Airbusses are typically 'clipped' at the corner.
Boeing 737 cockpit:

Airbus A320 cockpit:

Do mind that the eyebrow windows are not present on most Boeings. Current models no longer feature them and on some older aircraft they have been removed.

There is actually more distinctive feature in the images. The lower edge of the cockpit windows is more straight on Airbus with the last window mostly square while Boeing has the lower window edge distinctively angled. This is most distinctive on 737, but 757 and 777 have it too. It makes Boeing aircraft look a bit like frowning.

The only thing you need is the tail number. Google it :)
Otherwise, the wingtips are one thing that could be used to differentiate between aircraft having the same number of engines. From The Boarding Area blog:

Regular: These are the traditional wings. There are no tips at the
   end. They are seen on all Boeing aircraft except the 747 and none of
   the Airbus aircraft.
Blended Winglets: Winglets are a new type of wingtip that are used primarily to increase fuel efficiency. Boeing has winglets that can be seen on their single aisle aircraft – 737 and 757. All Southwest aircraft have wingtips. I have not seen wingtips on Boeing widebody aircraft, but I am told they are coming soon (Boeing 767 and 777).

Airbus recently announced that they will start adding winglets to their new A319, A320, A321 aircraft. They are calling them Sharklets. This will make telling them apart tough.
Wingtip Fence: These are the relatively tiny triangular winglets one can see in the attached picture. All current A318, A319, A320, A321 and A380 aircraft have these devices. This makes telling these aircraft from others real easy.

Wingtip Endplates: These are seen on A330, A340 and Boeing 747 aircraft. They are not upturned like winglets, but at an angle.

Wingtips are probably not the easiest way to identify an aircraft though. But a mention of raked wingtips would've been nice too.

'Wingtip end plates', as you call them, are just winglets. They may look different than blended winglets, but they are nonetheless winglets.

Wingtip end plates are usually called wingtip fences.

With so many great answers so far, I will try to add my "algorithm".
There are many tips to identify a passenger airplane model online. But I think this is still quite a challenge, as you do not always have a chance of a front view, or a perfect side view. So my point is that you do need to remember as many details as you can, so you will be able to tell the difference ASAP, at various angles.
Of course, I agree that it is important to learn by comparing Airbus and Boeing side by side, one by one, as some people already did, say comparing A320 to B737, A380 to B747. This is useful, but in reality, what you really need is to tell what "that airplane" is, yes "that one", not viewing two airplanes at the same time and tell the differences. This means the comparison method doesn't solve the problem for you, what you really need is to, based on each airplane’s appearance, in a short time, identify an airplane's model. So I encourage you to remember each model's characteristic details and its key features as accurately as you can.
Thirdly, remember, airplane models will be updated constantly, so the feature one model has now could be changed in its next generation. So be careful about its current sub-models and also be careful about its future updates. By the way, I am still not able to tell exactly from a several thousand meters away. Also, I am not a sound expert, so how to tell based on the engine noise, I don't know.
To repeat, it is important to memorize the details . Okay, let’s start!
Mostly, the airplanes we usually see in the sky or on the ground are: A320, A330, A350, A380, B737, B747, B777 and B787. There are also a few A340, B757 and B767 still flying. Canadian Bombardier CRJ series (typical engines close to the tail), Russian MC-21, Brazilian Embraer E190, and Chinese COMAC C919 are even fewer.
First, the A380 is the easiest one you could identify without a single doubt. Four engines, biggest one, double deck all the way to the tail. You won't miss it.

The B747, is also the easiest one. Four engines, double decker, but only a half double deck, goose head. The queen of the sky, magnificent beautiful and pretty darn easy to identify.

What, four engines but single deck? Well, that is for sure the A340. Very distinguishable, but you won't see the A340 very often now because it is not selling very well.

If the airplane you are seeing (no matter what angle you are viewing) is not one of these three, now comes the challenge. Let's talk about it now.
Seeing a chevron nozzle (that is, a jagged engine cowling)? If it is combined with super streamlined, flat and raked wingtips, that is a B787 for sure. Other Boeing models like the B737 MAX also adopt a chevron nozzle design, but the B737 doesn’t have a raked wingtip. You can also get confirmation by the B787's nose, which is more streamlined, bending toward the ground, making the airplane's bottom more flat in the nose section. Note that the B787 is distinct in its beauty and modern design, and you will marvel at it after a few looks.

Okay, then check the wing tip and APU exhaust. If it is a flat and raked wing tip and APU flat exhaust vent, then it is a B777 for sure. You can also be further confirmed by its 5 doors (including the one at the wing location), long fuselage or three row landing gear rear wheels. The B777 is larger because it is targeted for long range flights.

My next suggestion is to see if it is a B737. While the B737's cockpit window shape has a specific downward sharp angle at the side view (this is very unique as compared to other features even on all 737 families). The window of the A320 is more clipped, while the MC-21 is less clipped. But sometimes you don't have the window view. Then some suggest that the B737 has a point nose while the A320 has a fat nose. While I agree with that, but I feel that's not obvious to a layman and sometimes you can confuse it with the A330’s point nose unless you have a side view of the cockpit windows as well. I do suggest that you could go and see if you have view of engine's front. The Boeing 737's engine is not a full round shape. The bottom of nacelle has a more flat look than Airbus's. Then also check and see if you have good view of the big tail. The B737's forward tail has a V connection with its fuselage, while Airbus's doesn’t. You may also see if you can see the rear landing gear; if two rows, then it is for sure NOT a B737. Try all those above and if it's not working, then I think the best approach is to examine its wing tips. If you see a split wing tip, with main wing bending upward and an additional small winglet inserting from the downward, good, it is for sure the newest model B737 MAX. It features double-diagonal winglets that extend above and below the wing. If not I find telling them apart based on the wingtip is tougher and tougher now, because Airbus is introducing blended winglets into their A320 families like the current B737 NG. And Airbus called them Sharklet! Sharklet belongs to Airbus's A320neo family, including A319neo, A320neo, and A321neo. Traditionally for the A320 family, if you are not seeing a winglet, but a wingtip fence, which is smaller than a winglet, and very vertical at the tip of the wing, then these are for sure A320 classic families. And if you see a winglet which looks like putting an endplate at the end, not upturned like winglets, not that smooth transitioned, but at an angle, then they are the A330 (A340 and B747 also do this). Comparing to the A320 classic families, the B737 NG's winglets are much taller, aggressive and with a smooth transition.
While both the A350 and the B737 NG have blended winglets, it's how to tell the difference; easy, the A350 is much larger than the B737 (larger engines, wider spans, longer, etc.). The B737 is for short to medium range flights, just as the A320 family, B737 and A320 have only two doors. Note, here we are not counting any small emergency doors at the wing position. If you see a small airplane with blended winglets, and you can't tell if it is Airbus's Sharklet or Boeing's winglet, then you need to check its engine nacelle front shape, cockpit window shape, forward tail connection, etc.
The A330 and A350 are close. Being larger and having more doors makes them distinct from A320s. The A350 is meant to compete with the B787. It is also targeted for long range flights, while the A330 is targeted for medium to long range flights, so in general, the A330 is slightly smaller than A350. Then note, A350 has a "spectacle" on the cockpit window and its wingtips are beautifully more smoothly transitioned than the A330. The A330's winglet is sharply transitioned. The A350 looks very non-Airbus. Elegant and beautiful, I think it is the most beautiful one in Airbus's family.
Besides, we could also check the first top antenna. All Airbus's first top antennas (VHF1) are closer to the nose than Boeing's and it is also taller than Boeing's first antenna.
The end.
To-Update: need more figures...

Things have been greatly complicated in the winglet department. The 737 NG has a split scimitar option similar to the MAX, the A320ceo can have sharklets like the neo, the A330neo has winglets more like the A350, the E190 and A220 have similar winglets to the sharklets, but not the E190-E2...

One thing that comes into my mind is the winglets (when present on newer aircraft). Airbus uses those that are triangular wingtip fence whereas boeing uses the blended winglet.
Also the shape of the nose can give you an indication. On Boeing they are more sharp and protruded whereas on airbus the design is "softer" and more rounded..
Check pics of a 737 and a A319 and you will notice the difference

The new winglets ("sharklets") available on the A320 family are much more difficult to distinguish from Boeing and Embraer winglets.

In the past, the presence or absence of winglets alone was a pretty good clue.  Now that they're becoming more standard it's not as good of a guide anymore.

True. As the a330 has winglets that are similar to boeing ones. But if you watch carefully the airbus ones are more sharp edged whilst the boeing ones are more curved where they connect to the main wing.

The confusing ones are twin-engined, wide or narrow bodied.
Narrow body, (single-aisle): 737, A-320, 757, (Also Embraier Em-170...195, Sukhoi 100, Bombardier C class, Tu-204..)

Nose shape is useful. Boeings tend to be more pointed, as shown above.
Airbus had little 'winglets' that go both above and below the wing on their older products. Boeing winglets are much taller, with curved transitions from wing to winglet. The latest 737 options are double-diagonal winglets that extend above and below the wing.
737s have very short main landing gear, so their engines are mostly ahead of the wing. Their main gear have 2 wheels (as shown above) and the outside of the outer wheel and tire are visible from below, no doors over them. Lots were built without winglets, but retrofits and new airplanes have tall winglets that curve up above the wing. The latest 737 winglets are inclinded, above and below the wing, like half an "X".
Exclusive operators include Southwest (USA) and Ryan Air (Ireland).
A-318, A-319, A-320, A-321, are different lengths, short to long, narrow bodied, similar in size to the 737 family, but have longer landing gear legs, and their engines are under their wings. Almost all built with with small, flat, winglets, above and below the wings. Exclusive operators include Jet Blue, Virgin America
757 family are narrow-bodied twin engined, with unusual cockpits. They share the cockpit design of the wide-body 767, higher numbered but designed earlier. 757s have deeper fuselages aft of the wing than in front of the wing. Many are being retrofitted with large winglets.

Wide body, (double-aisle): A-300, A-310, A-330, 767, 777, 787.
more later.
Three or four engine airplanes are really pretty easy:

A-380s, with two decks, full length, are Airbus products.
747s, with small to medium upper decks behind the cockpit, are Boeings.
727s, with three skinny, low-bypass, engines, are Boeings.
Tu-134 and 154 are three-skinny-engined, narrow bodied, with distinctive fairings on the trailing edge of the wings into which the main landing gear retract.
DC-10 / MD-10 / MD-11 Are wide-bodied, have high-bypass engines, with the middle one mounted in the middle of the vertical fin/rudder. FedEx operate them, but not exclusively. The USAF use some as tankers.
L-1011 are wide bodied, have high bypass engines, with the middle one mounted inside the back of the fuselage, like a 727, Tu-154, etc. Orbital Sciences operate one as a carrier for satellite launches. The RAF used some as tankers, nearly all are retired now.

If you look at the tail point exhaust (next to the stabilizer), Airbuses have them positioned horizontal to the body and ended in a round opening.
In the case of Boeings, it is placed slightly below the horizontal cross-section of the fuselage and has an oval opening.

Long answer here: https://alliknowaviation.home.blog/2019/03/30/37/
The easiest trick I use to distinguish between an Airbus and Boeing is cockpit window shapes, much similar to as mentioned by @dotancohen.
Airbus

Boeing

There are some exceptions though. Check out the 'Long Answer' link.


text:
Why must one wait 48 hours to fly, after scuba diving?
Is this rule even true and why is it? I'm going to assume it has to do with pressure but can someone give me a good explanation?

I misread "after driving" and I thought WTF?!?

Trivia: Thhis is the plot to an episode of House

I seem to remember 24 hours from my diving classes.

Some diving watches will show "no fly" for some time after diving on the watch face. To remind the diver about that rule.

Yes It's true. And yes it's related to pressure.
Have you heard of "The bends" or decompression sickness?
As divers go deeper the pressure increases. The longer you dive and deeper you go the more nitrogen is absorbed into your blood as a dissolved gas. As you return to the surface the pressure reduces and the nitrogen reverts to a gas.
This decompression needs to be done slowly so the nitrogen can pass back out through your lungs.  If you ascend too fast the nitrogen can form bubbles in your blood (Just like when you open a bottle of coke.) which can be painful and possibly fatal.
Even once you have finished diving you will still have residual nitrogen in your blood which will take time to release.
Air pressure reduces further when you fly. If you finished a dive and got in a plane the rapid climb in altitude would result in a drop in pressure similar to a rapid ascent while diving. So you would run the risk of getting the bends.
Small planes aren't pressurized. But even pressurized planes might not maintain atmospheric pressure when flying.
This topic should have been covered if you have taken a dive course.

This all makes perfect sense but, since the question includes an explicit "Is this rule even true?", it would be good to include a couple of references, too.

It's of course an imprecise rule. The partial nitrogen pressure in your blood determines whether you can fly, and that pressure depends on the dive profile. Then there's the nitrogen buildup in tissue (especially fat) which further complicates the analysis. Some background. 48 hours is just keeping it safe for normal diving.

'migt not maintain...' In practice, planes are almost never pressurized to 1 atm, but usually about 0.75 atm.

Waiting 48 hours seems overkill. The French Federation recommends 12 hours after a single dive, 24 hours after two or more dives (See Federico's answer for PADI).
Dive computers compute this time from your actual dives; I have never seen more than 20 hours displayed.

+1 This topic should have been covered if you have taken a dive course.

Yeah, passenger aircraft are normally pressurized to pressures that are equivalent to 6,000-8,000 ft. altitude. Keeping surface pressure at 35,000 ft or more would require a very strong fuselage, which would weigh a lot more and, therefore, waste a lot of additional fuel.

It's interesting that I was taught 48 hours when I took an aeronautics exam which was in France, @ChristianLescuyer!

@ChristianLescuyer I think that's the point - make it absolute overkill to make sure. If 20 hours is the longest time you've ever seen a dive computer come up with, then 24 hours is still within a reasonable percentage of that. Rather than teach people a more complex rule involving the number of dives, how long they lasted, etc. it's just simpler to come up with a single value for people to remember that's definitely going to be safe in pretty much all scenarios. 48 hours is the highest value you've ever seen, doubled, rounded up to the nearest full day. Definitely safe.

"nitrogen is absorbed into your blood as a liquid"  would better be described as "as a dissolved gas".  Liquid nitrogen would, I hope, never be associated with bodily fluids.  See last scenes of Terminator II:  Judgement Day

@ChristianLescuyer - The dive computers use algorithms to determine the 'likely' remaining dissolved nitrogen in your body.  It likely uses multiple ingress/egress profiles to represent the different tissue types in your body.  These models are based on theories and can be mitigated by a variety of factors including hydration, drugs, %age of body fat, muscle tone, etc.  They each have their own safety margins and can vary widely.  The 48-hour rule is meant to be reasonably certain one shouldn't experience ill effects from flying (which might include emergency cabin depressurization.)

The Benz? Ah, what a nice car.

Thanks to all for adding detail and clarifications to my answer. I'm a bit limited using a mobile app.

Nitrogen is absorbed in virtually all tissues in your body, not just blood. (although blood is the transport medium that moves nitrogen in and out of your body).   Dive computers use numeric models to track how quickly nitrogen is absorbed into and released from about 13 different major tissues (muscle, fat, liver, nerves, etc...), and then show the most significant/most dangerous level.  Some organs absorb nitrogen more readily than others, and release it more slowly than others.

My own surface interval is now over seven years. Way too long...

To add a reference to Peter's answer, from PADI, one of the institutions that releases Scuba diving certificates:

Flying after Diving Recommendations
For Dives within the No-Decompression Limits

Single Dives: A minimum preflight surface interval of 12 hours is suggested.

Repetitive Dives and/or Multi-Day Dives: A minimum preflight surface interval of 18 hours is suggested.

For Dives requiring decompression stops:
A minimum preflight surface interval of greater than 18 hours is suggested.

These are "suggestions" by diving organizations, for divers who want to fly.  How about suggestions from flying organizations for pilots who want to dive?

@User58220 is a bit more common for a diver to have to take a plane than the other way around. Also, if you are a diver that wants to fly as a passenger you will not get the information that is given only to pilots. With the current method all interested people are informed (all people that scuba AND fly have to take the scuba course), with yours many are left without information (not all people that scuba also take pilot lectures).

This doesn't answer the question, which is why one should wait before flying.

@DJohnM: "First, get out of the airplane."

It's not just blood, but soft tissue generally.
48 hours is extremely conservative - recreational (single tank of air, 40metre maximum depth) diving guidelines say only 24 hours.
Don't forget altitude (lakes at > 300metres) diving also affects No-Decompression Limits.

When diving, your body absorbs nitrogen gas. As you ascend towards the surface, your body releases that gas.  If it is released too quickly, you'll get "The Bends".
Modern dive computers model the body as a series of different tissues: fat, muscle, liver, cartilage, brain/nerves, bone marrow, etc. Different dive computers will model different tissues, typically between 8 and 20 tissues.
Each of these tissues absorbs, retains, and releases nitrogen at different rates. The dive computer will track a number of different tissues, but show you only the most critical of the tissues (eg, the one that will take the longest to off-gas to a safe level). Depending on how long you've spent at various depths, the critical tissue will change.  This is guidance for just returning to the surface safely.
Other factors affect each individuals off-gassing rates, such as BMI, general health, specific metabolism, and alcohol consumption.
When you later fly, you're going to even less pressure, and your tissues will off-gas even more.  If you're flying in a typical pressurized commercial jet, you'll be at around 8,000 ft pressure-altitude.  But if you're in an un-pressurized charter flight, you could easily hit 10,000 - 16,000 feet.
Many people have trouble understanding that a "small" plane will actually hit a higher pressure-altitude at 14,000 feet, than a "big" jet at 30,000 feet.
If you haven't been doing extreme diving, and are flying pressurized, then 24 hours is safe for a normally healthy person.  If you're flying un-pressurized above 10,000ft, then 24 hours may be safe.  But because The Bends depends on many subtle factors and is unpredictable, 48 hours is considered much safer.
This leads to the typical dive planning advice that you should begin with your most aggressive (deepest, longest) dives first, and use your easier dives later in the trip.  Your body can continue to off-gas from your most deepest dive even during your subsequent, shallower dives.   This is a little contrary to human nature, as many people want to begin at an easier level, and get more aggressive with each dive as their comfort level grows.  But that only sets up the body for a high N2 load, adding more and more with each dive.
If you have any aggravating factors, such as poor health, significant fat (BMI), alcohol consumption, or a prior incident with decompression, then I would avoid the more aggressive dives and plan on as much time as practical before even a pressurized flight.   For many recreational divers, this simply translates to "no diving and no drinking on the last day of a trip:  Just go sightseeing, or stay on the beach"

Credentials: Certified NAUI, PADI, and SSI; Trained Recuse Diver; Experienced with Deep, Night, Dry Suit.  (but mostly retired and above the surface now).

By every recommendation I've ever seen 48hrs is extremely liberal for recreational divers.  I went on my first dive trip with my parents when I was 12 years old and our rule was to quit diving at least 24 hours before your flight.  It is related to pressure and it all depends on how much time you've spent and at what depth.  The deeper you go, the quicker you build nitrogen in your tissues and the more time you'll need to allow for that nitrogen to dissipate.  If you spend enough time at depth there is even a recommendation to make a safety-stop during your dive (typically 5-15 minutes at about 30ft for recreational diving) as a best-effort to avoid the bends and surface decompression.  The U.S. Navy created classification charts that indicate your relative nitrogen absorption.  Here are some resources hosted by NOAA, one of them being a PDF specifically covering this topic.
http://www.ndc.noaa.gov/dp_forms.html
http://www.ndc.noaa.gov/pdfs/USNDeco3.pdf


text:
Is there a flexible "cloth-like" material that can be used as "skin" for a non-conventional wing?
I am in the very early stages of playing with the idea of an ornithopter derived aircraft. The intention is to be able to scale the resulting aircraft, to passenger (or useful payload) carrying capability.
Is there any material that is simultaneously light - but also strong enough to carry the weight of an aircraft (and it's payload)?

Aircraft fabric would be too heavy for a person carrying homebuilt orni-contraption.  What you are looking for is 3M Mylar/Tedlar.  It has been used as a covering material for a number of ultralights like the Lazair for many years.

can a material like spandex be used instead of 3M Mylar - as the former is more readily available, cheaper and also crucially has great elasticity.

It has to be impermeable.  And you don't really want elasticity, just flexibility.

Thanks for your prompt response. Am I right in concluding from your answer, that - if I'm able to find impermeable spandex (i.e. airtight), such material would be suitable for use as wing skin (i.e. in terms of weight/strength)?.
Does such material (airtight spandex) exist?

As I said, stretchy=bad.  Spandex would be no good.

No idea.  You need to select some different products and test them.

Yes, cloth has been used for wings since the first powered flights. It is usually doped with a resin to make it less permeable to air.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
How is fuel removed from a light aircraft if required for weight and balance?
With small, light aircraft, weight is always a consideration. Say you had a four-place single engine aircraft that you wanted to take yourself and three of your (non-FAA-standard weight) friends, but also at the time you get to the aircraft, you discover the tanks have been topped up.
Is there a standard way (or best practice) for removing fuel from the tank so you could carry all your intended passengers (provided you don't actually need all the fuel to get to your destination) and stay within the gross weight limit.

You can use a siphon and a tank to transfer some fuel out through the filler cap. Really depends on the tank configuration.

Fuel is expensive these days. Once you remove it from a tank it can’t be used.

Several ways.
First, determine if there really is a weight and balance discrepancy with a proper weight and balance calculation.  If the aircraft is overweight and out of CG, can you simply remove some baggage or other non essential cargo in order to return to the weight and balance envelope?  If so, then do this, perform another weight and balance calculation and if it is within the envelope for safe flight, problem solved.
If the airplane is within gross weight but outside of CG, can you move some payload around to another fuselage station e.g. A fwd baggage compartment, etc so the CG can be moved within limits?  If so, do that, then do another weight and balance calculation.  If you're within the envelope, you're good to go.
If you cannot lose or rearrange any other non essential baggage in the cabin and you're still overweight, first verify that removing said amount of fuel will 1) keep the aircraft within weight and balance limits and 2) will be enough to safely complete the flight with the required onboard reserve, you have two basic option, drain it out, siphon it out or burn it off.
Before beginning this exercise, make sure you have an accurate means to gauge just how much fuel is in the tanks currently.  The tanks are generally placarded with their useable capacities or this will be listed in the Pilot's Operating Handbook for the aircraft in question.  Sometimes takes also have pre-marked indicators inside the tanks where a known quantity of fuel will level with these markings or metal tabs.  Some firms also sell gauges for determining fuel quantity in the tanks; just make sure you have one that's approved by the manufacturer for your make and model airplane.  DO NOT use the cockpit fuel gauges in a light GA aircraft to determine onboard fuel quantity; by regulations these are only required to be accurate when they read as empty (how convienent).
You could drain fuel from the tanks through the sumps, generally located under the wings.  A tool with a rod would be required to open the valve on the sumps and you could use a tube with a funnel on it to collect the fuel into a container.
Siphoning the gas may be easier using a tube through the gravity filler ports on the top of the wings.  One downside is that these filler ports are often on the highest part of the tanks and may require a long hose to siphon the gas.
The third option is simply to burn off the gas by making a flight prior to the arrival of the passengers for the planned trip.  Using the takeoff, climb, cruise and power settings charts in the Pilot's Operating Handbook, you can make an accurate estimation as to how long it will take and what power settings to use to consume the excess fuel prior to loading for the trip.

Instead of using the sump under the wings, which will require some sort of custom made rod, try the sump under the engine.

This answer is correct, but the beginning half should be removed. The OP is very clear that s/he is interested in ways to drain fuel, not ways to shift weight around or otherwise get the plane into balance.


text:
What is the benefit or motivation for having Class G airspace?
I'm looking at the differences between Class E and Class G (AIM 3-2-6) airspace in the United States and I get that there are different flight condition requirements depending on AGL/MSL altitudes, but if you're outside a Mode C veil and around an untowered airport (So you don't need a transponder, don't need a radio), why not make everything Class E?
I think the answer came to me while writing this out...is it this simple?
Does Class G exist because for whatever reason (Radar services not available, technical reasons, etc.) ATC is unable to provide IFR separation services in that area, so it has to be deemed "uncontrolled?"
Is there some other reason why Class G exists besides being a catch-all?

Class G airspace (uncontrolled) is that portion of
  airspace that has not been designated as Class A,
  Class B, Class C, Class D, or Class E airspace.

Like is there some benefit that you get when you're in Class G instead of Class E? The only thing I could think of is that the flight visibility is less than 3 miles but more than 1, it's daytime and the cloud deck is lower than 500 feet AGL (since you just need to be clear of clouds and 1 SM visibility), you could take off VFR in Class G and you couldn't take off in Class E. (However, you'd still have to be sure not to be engaged in Careless or reckless operation (14 CFR § 91.13).)

Are you asking for FAA only? Cause in EASA, it's really just there to allow VFR traffic to fly in not-so-nice weather...

I'm asking FAA/United States only.

My designated pilot examiner explained it to me like this: "When the government decided to take over all the airspace, they left a couple pieces for the guys complaining about "the man" being in their hobby, so they said "here's a piece, and here's a piece, etc" to appease those people".

It's so Superman can fly around at low altitude without having to carry an airband radio.

You can also fly Ifr without a clearance in G.
Remember that the entire purpose of airspace is to keep us from hitting airliners. It is not about keeping GA planes apart. When you look at wx mins that is clear. The FAA gives us the freedom to fly around in low vis so long as we stay out of the areas you'd find airliners.
-Robert CFII

"You can also fly Ifr without a clearance in G."  Do you mean in IMC, or under IFR?

Airspace classifications above G exist to protect commerce. (The FAA exists to protect commerce.) That is primarily IFR operations, as well as carriers and others that utilize ATC services.  The protections include around congested airports (towers, radar service areas) and airspace for route structures.
Just 50 years ago, there was much more uncontrolled airspace in CONUS. Air routes (Victor airways) were protected, and airspace moved to higher classification levels, and along with it NAVAID facilities were implemented, better radio coverage for FSS and ATC, as well as more terminal and enroute radar service areas.
FAA Order JO 7400.11B is an example of the FAA issuing orders classifying airspace for the purpose of serving airports and their instrument approaches. Other airspace, such as Class A was defined by broad regulation.
When I was first instrument rated, it was possible to make substantial trips IFR, without a requirement to file flight plans. The airspace was uncontrolled, and little or no ATC services were available. Flight plans could be filed for search and rescue, but separation services were not available.  FSS stations and their remotes (RCO) were a primary link for IFR operations in uncontrolled (now Class G) airspace.
It is noteworthy that radar coverage is not necessary for airspace to be classified as controlled airspace.  Rather there is the need to be able to control that airspace to assure the separation and provide weather services for aircraft.  It is true that some airspace, for example, Class C, is defined by the existence of radar services.  If radar goes down, then class C reverts to Class D and class E.  But in general the classification of airspace is done to support the operations and the services rendered in that airspace.
https://www.faa.gov/air_traffic/nas/nynjphl_redesign/documentation/feis/media/Appendix_A-National_Airspace_System_Overview.pdf
https://www.faa.gov/documentLibrary/media/Order/JO_7400.11B.pdf

All airspace was originally uncontrolled.  Then planes started crashing into each other, especially in bad weather, and someone got the bright idea of having controllers to track where planes were and keep them separated.  Yay!
Those controllers expect to be paid and need radar/radio equipment, though, so it was only done where there was enough danger to justify that expense, e.g. around busy airfields.  As planes got faster, a low-altitude speed limit was added so that new danger was limited to high altitudes, which are cheaper to control due to physics.  But traffic kept growing, so they kept adding more and more radar/radio coverage to enable control at lower and lower altitudes.
There's still a limit to budgets, though, so most of the last bit near the ground remains uncontrolled--now called class G.  In some countries, it's actually a rather large part of the lower airspace because they simply haven't thrown as much money at radar/radio coverage as others have, and that's often (but not always) linked to lower traffic density and correspondingly lower collision risk.

Class D and above require good weather, and permission, to enter.
Class E airspace requires good weather only.
Class G airspace requires not much. That's the benefit/difference of Class G.

Class A also requires an IFR flightplan to enter.


text:
How do I convert true heading to magnetic heading or the other way around?
Is there a method by which the conversions can be achieved on the fly by using a mathematical formulae and not using specific area charts?

Short answer: Yes there is, but it doesn't always work, it involves a LOT of math, and you're probably not gonna like it.

Medium Answer: Most flight planning software incorporates a True/Magnetic heading conversion system (ForeFlight for example).
Using an electronic flight planning tool like this is probably your best bet if you want an on-the-fly determination made for you.

Long answer:
International Association of Geomagnetism and Aeronomy (now there's a mouthful!) periodically publishes what's known as "International Geomagnetic Reference Field (IGRF)".  It's a mathematical model of Earth's geomagnetic field, which you can work from some exceptionally complicated trigonometry (or just look up the coefficients in a table which is what most sane folks do).
If reading text like "Mathematically, the IGRF model consists of the Gauss coefficients which define a spherical harmonic expansion of the magnetic scalar potential." makes your eyes roll back into your head (I was a math nerd and it makes MINE roll back!) using the model is probably not for you.
If your eyes are still focused on this text however, click on through to that link way back at the top of my answer and you'll be rewarded with a formula you can plug into your trusty calculator.

Great answer! I like the "health warning"  (sic!):" "BUT INAPPROPRIATE USE COULD SERIOUSLY DAMAGE THE CREDIBILITY OF YOUR RESULTS".

@yankeekilo "Don't come and blame us if you fly into a big metallic mountain. It's just a model!"

How in tarnation did this answer get 19 upvotes over the other answers?!!!  More than making eyes roll back in your head, anything having to do with "Gauss coefficients which define a spherical harmonic expansion of the magnetic scalar potential" is incorrect for the question that was asked.  Simply add or subtract magvar is the right answer for performing "on the fly" conversions.

Magnetic direction and true direction are interchanged with the help of variation. Rule of thumb is:

Variation west magnetic best ( more than true)
Variation east magnetic least ( less than true)

Or
MD + VAR EAST = TD
MD - VAR WEST = TD

While a lot less elaborate, this simple answer is probably more helpful in most situations. When taking FAA tests that involve making this calculation or teaching the basics of flight planning, remembering "east is least" to do the calculation is what you need to get the job done.

@ryan1618, "Less elaborate" was precisely what the question asked.  (see my comment on the answer that had 19 upvotes...)

True
Virgins
Make
Dull
Companions
At
Weddings
That mnemonic device reminds one how how you go from True to Magnetic (compass heading, course, etc).
Take the True heading, apply magnetic Variation to get Magnetic heading then apply Deviation - and there's your Course (or the number on your compass that you will steer by).  Add West(Subtract East) means that if the Mag Var is to the west, you add to True, if Mag Var is East, you subtract from True.
Politically incorrect but 45 years ago that’s what we were taught
(It is an import from maritime navigation).

Please decode the saying, for those who aren't already familiar with it.

Ken, I filled in the details, which you and I may have learned from the same Nav instructor, or the same vintage of Nav instructor, over 40 years ago.

For this,  you use what the others have suggested, MH = TH - Variation West or MH = TH + Variation East.
After this you have to apply the deviation to the MH. i.e is your compass accurate or is it off by a few degrees. For Ex: there will be a sticker showing what correction should be applied to your magnetic compass heading.+/- 3 degrees etc.. like 270 is off by 3 and will show 267.

MH = TH - Variation West
or
MH = TH + Variation East
For practical purposes, I have read a METAR and it states in writing that the wind is 355@10.  I look at my sectional and see 7 W.  355-7=348.  348 will be the approximate direction of wind (within 2 degrees) @ the time of the observation.

Wow.. thank you for explaining how to use TH MH with a sectional. Instant practical knowledge.

If you prefer a metaphysical answer, visualise the true North Pole and the magnetic North Pole being two points on a sphere, if you happen to be on the great circle that passes through both then they will be in exactly the same direction (or exactly opposite if you happen to be in between them).  If you’re comfortable visualising the difference between them at other points on the globe then you may be happy to estimate in your head, probably until you learn that the magnetic South Pole is not quite diametrically opposed to magnetic north.


text:
What factors/equation(s) are used to relate aircraft weights and cruise speeds?
With the recent coronavirus issues, there have been lots of empty seats flying around, especially on long haul flights.  I checked the seat map for a recent SFO-SYD flight (777-300) and there were 224 empty seats out of 340 available.  This obviously means the aircraft was much lighter than usual for this run - a quick estimate gives me 25 tonnes less weight just in people/baggage.  To get to the actual question, when doing flight planning what equations or other mathematical factors go into computing how this impacts the climb rate and cruise speed for the flight?  I'm familiar with the concepts of induced drag but I don't have an intuitive or empirical understanding of the actual impacts the reduction in weight would have.
Does taking 25 tonnes of weight from the airplane move the climb rate by 100 ft/min, or 1000 ft/min?
If we have 25 tonnes less "cargo", this means we also need less fuel to lift and carry the aircraft... but how much less?  How do we compute the fuel savings for the lower take off weights?  I know there have to be formulas (or at the very least charts/curves) but I didn't have much luck finding them.
Is the cruise speed meaningfully increased by the lighter weight?  Or does it just require less thrust (i.e. lower fuel burn) to fly at the same cruise speed?
I realize these are framed as discrete questions, but I think they're all inter-related in a way that's close enough to put them into a single post here.  Moderators feel free to disagree  ;-)

At some level I should have known to just look for anything Peter Kampf had answered... not sure why the search didn't find some of these, but here's a sample for anyone else who stumbles across this: https://aviation.stackexchange.com/questions/8652/how-can-i-calculate-the-fuel-consumption-of-an-airbus-320-200-at-various-loads/8654#8654 https://aviation.stackexchange.com/questions/8219/where-can-i-find-accurate-information-about-fuel-efficiency-of-airbus-vs-boeing/8221#8221 https://aviation.stackexchange.com/questions/3748/why-1st-and-2nd-class-but-not-fast-or-slow-airplanes


text:
What can I do if I cancel IFR when I shouldn't have?
Standard procedure for T-38 training is to cancel IFR in the MOA and fly back to the base VFR.
A couple days ago I canceled in the MOA out of habit and instantly realized that I was above a broken layer between my and the base.
Luckily the other side of the MOA was clear, and I used the extra distance for a more shallow descent than I'd have otherwise done.
But it got me thinking, could I have asked ATC to get me back on my previous clearance?  Being a military training flight, and military controllers I'm sure they would have been super chill and helped me out regardless.  But in the civilian world what would be the best way to handle this situation (other than "don't cancel IFR")?

You can call FSS or center and pick up a clearance in the air. Here is an article about it.

This is as a comment since I retired in 1999, but my suggestion is do do the simplest thing and say something like, "Center, N1234, I cancelled IFR prematurely. Request clearance to....., sorry about that." Pop up clearances are commonly given.

@Terry - I retired in 2012, but I still teach ATC at my old facility (I could be a loser!), but your comment also still works as an answer. Also, it's a piece of cake these days to get a new flight plan into the machine. In EDST, the canceled flight plan will stay in your list for a bit, and you can recall it and re-enter it in a couple of trackball entries.

Since you used the term MOA, I assume you're in the US.
Within the FAA ATC software (not sure about DOD's), the only data difference between IFR and VFR Flight Following is the flight plan altitude. For instance, IFR might have a filed altitude of "060" (6000ft) while VFR FF might have a "filed" altitude of "VFR/065" (6500ft). When you cancel IFR, the controller amends your altitude and leaves the rest of the data unchanged. They don't actually delete your flight plan unless you cancel radar service. Therefore, to resume IFR, all they have to do is amend your altitude again.
Obviously, IFR and VFR are very different procedurally. When you canceled IFR, your existing clearance was invalidated, so you'll need to get a new one. However, since they should still have all your previous data available, it'll probably be similar. But it's less work for them than a normal pop-up IFR.

Based on OACI standards (which FAA follows) you couldn't re open an IFR already closed; but while you are on air you can fill and ask for a new flight plan, asking to using same data of previous flight plan.


text:
How "over-engineered" were old planes and how "over-engineered" are they now that we have more advanced testing and research options?
I've seen several times on shows on television, and even talking to older pilots, where they will tell me that old planes were "over-engineered" to add some margin for errors on the designed engineering calculations. Of course, no specifics are given, and so I was thinking in this current age of super-fast computers, better wind tunnels, new software tools, and decades of actual experience, when people design new planes, how much of a margin to they design into them now?
It seems like confidence in computer modeling seems to be pretty high, and "over-engineering" seems to have typically meant making things stronger than they "need" to be, more reinforced elements, all things which add to cost and weight of an airplane which are factors that are at odds of profitability and performance.

What do you mean under "old"? You can't say planes before 1920 were over-engineered, as they ware barely being able to fly short distances before their engine stalled or their wood and canvas wings snapped.

@vsz I think it would be fair to consider the large numver of Cessnas, Pipers, and similar still being flown that were built in the 1940s-1970s “old”, and everything after that (which has been able to take advantage of computers, wind tunnels, etc.) as mid-aged or new.

@vsz I don't think that pre-1920 airplanes were, as a class, as incapable as that: Many of the aircraft of WW1 (1914-18) were capable of acrobatics and had ranges of hundreds of miles.

Consider the amount of engines used on a plane - older planes tended to have more than necessary (4) simply because they wanted the plane to keep flying even if two engines failed, vs modern planes that will generally keep flying with only one engine out (with two engines normally)

@WayneConrad : if you look at the statistics, you'll see that almost half of the aircrew casualties were from accidents instead of enemy fire.

@SlippD.Thompson I guess I was a little vague. I was thinking the 1930-1970s. What prompted the question was Smithsonian's series on "Planes that Changed the World" where they featured the DC-3 and they talked about how it was "over-engineered" but just threw that term around without definition.

You are right that older designs used (and needed!) higher safety margins. I would, however, not call this over engineering. But that is besides the point.
The biggest advances have been made in materials! Not just new alloys, but much better quality control in manufacturing. The aluminium sheets and slabs you would get from a factory 50 or 80 years ago had much greater variations in local strength, and in strength between different production batches. The same goes for fasteners, forgings, whatever. Computer-controlled manufacturing and the relentless effort to improve quality and consistency have made this possible. Only wood is still the same as it was then, but Basil Bourque is right: Processing the wood has also improved by leaps and bounds.
But now consider what we do when we stress a wing: We use the maximum static lift coefficient, add aileron deflection and use that bending moment at v$_A$ to size our wing spar. We do this by applying a safety factor of 1.5 (§25.303). For parts with a more error-prone production process additional factors need to be added (see §25.619).
And now consider what happens in the real world: The dynamic maximum lift coefficient, which does not obey the rules and regulations imposed by the FAA, will easily be 1.3 times bigger than the static lift coefficient used for stressing the wing spar. In tests up to 150% have been achieved. Makes you think how sensible that safety factor really is. While we understand the static loads very well (because they are easy to study), dynamic and fatigue loads still happen to surprise the engineers from time to time. A little extra margin sometimes can be quite useful.
On the Eurofighter the structural safety factor was indeed reduced to 1.4 with the reasoning that more precise modeling and simulation would allow to use a smaller margin.

If you want to add a real life example of exceeding the 1.5 safety factor in a dynamic situation, you could refer to American Airlines 587 (pdf). Page 61 shows the limit and ultimate design loads, and the loads created by the crew inputs.

@DeltaLima: This is a good example of dynamic loads which would have been almost trivial to calculate, but were not covered by regulations. The safety factor is fine, what was missing was the consequence of repeated inputs at the eigenfrequency of the yawing motion. Airbus complied with the letter, but not the spirit of the (flawed!) law in this case. A good engineer could and would anticipate the higher loads.

Is that really something that could've been anticipated? Of course you can assume that a pilot will do just about anything to the aircraft, but how realistic is it given that pilots are supposed to be highly trained and not put these kinds of loads on the airframe? Plus modern Airbuses (what's the plural of Airbus?!) flying in normal law won't allow these kinds of input to reach to the control surface anyway.

I'm just wondering, obviously pilots are humans and make errors, but is the rate of that so high that the safety margins need to be increased so that pilots don't accidentally tear their planes apart in mid flight? It doesn't seem to happen very often from what I can tell.

@JustSid: Yes, most pilots know what they are doing. On smaller aircraft without power boost they feel from the control forces what they do to their aircraft. But some are overconfident and do not realize how close they operate to the limits. Like the British certification pilot for the Do-228 who allowed the trim to run all the way forward and then could not pull out from the resulting dive. You are supposed to react within 1.5 s from the start of the trim malfunction. If you wait too long, that's it. His last words were "help me on the stick!". This is a general characteristic of complex ...

@JustSid: ... technical systems that they are more complex under the hood than most operators assume. If they don't stick to the rules, they can die, be it in a nuclear power station (Chernobyl is a classic case) or in aircraft. Only very few people have the detailed knowledge to define the rules, but they don't operate the equipment.

@PeterKämpf I totally understand where you are coming from, and I understand over confidence, but is this something that should be fixed by making the airframe stronger? Obviously I don't want to suggest to overburden the pilots with things they have to keep in mind, but to me as an absolute layperson it seems like flying the aircraft within its constraints shouldn't be too hard, especially since most aircrafts these days make it back to the ground in a single piece.

As you can tell, I'm a programmer, I would rather the user gets fixed than the engineer having to do it :p

@JustSid: you can always beef something up, but you can also install discipline into the operators and inspectors to operate optimized equipment safely. Where do you draw the line? Would you make the wing and flaps strong enough to extend flaps at top speed? The beancounters at the airlines would not buy the product. We arrived where we are by trial and error and have a situation that is 99.99% safe already.

Only wood is still the same as it was then. As for wood, hasn’t even that improved dramatically in recent decades as well with modern fabrication of plywoods? Better process control, better shaping, better glues and resins? (I am no expert)

@PeterKämpf -- of course, making it so that misoperation is benign is the best solution -- for your flaps case, instead of having something break if you exceed Vfe and try to extend the flaps, making it so the flap actuators stall/are pushed back by the airloads (i.e. airload-driven flap load relief) would be a better solution, no?

@Basil Bourque LVL wood also improved in size/length ; There were also huge improvements over the quality of massive woods : computer automated imaging, automated x-rays controls etc..

Thanks for the great answer. I guess that's why I used the term over-engineering in scare quotes, I knew there was a better word or phrase for what I was trying to ask. When I think about it, I was probably asking about what you called higher safety margins and @aeroalias called more conservative load estimates, but you definitely answered my question.

@BasilBourque You are right, processing has improved tremendously. I was thinking of the raw materials.

The legal "over engineering" factor has not been changed since 1970:
FAR Part 25, section 303:

Unless otherwise specified, a factor of safety of 1.5 must be applied
to the prescribed limit load which are considered external loads on
the structure. When a loading condition is prescribed in terms of
ultimate loads, a factor of safety need not be applied unless
otherwise specified.
[Amdt. 25-23, 35 FR 5672, Apr. 8, 1970]

Over-engineering is a concept in value engineering, not aircraft design. In general, over-engineering means making the design more robust or unnecessarily complicated than necessary. In that sense, I would say that aircrafts are/were rarely over-engineered. Older aircraft were more conservative in determining the loads.
In structural design of any aircraft, the designer tries to make the aircraft safe in a number of steps:

Safety factor (which for transport aircraft, is 1.5)
Conservative material properties
Conservative loads.

The process followed now is pretty much the same; however, the engineers are able to determine the loads acting on the structures to a greater precision compared to the earlier times and are able to optimize the structure accordingly. The safety factor and taking of conservative material properties have not changed much.
Though one can say that, for example, making the wing spar thicker than necessary to account for unforeseen loads is good from structural point of view, it is bad from performance/weight/fuel consumption views. The problem is that if the designer knowingly pads the safety factor more than necessary, he's actually reducing the aircraft performance. As a result, over engineering (more than design or regulatory requirements) is plain bad design and is not recommended.
In short, the process haven't changed over time- it is simply that the tools have improved and more data is available for use.

There are other issues here- aircraft like F-35 Lightning II are repeatedly called over-engineered, but the point is that they were designed for the given specifications. For example, F-35 was required by design to perform the tasks of a number of aircraft and as such is having a significantly difficult engineering period.

+1 for pointing out the difference between "over-engineering" and safety margin. All mechanical and civil engineers dealing with safety-critical systems use safety margins in their calculations. The aerospace industry is no different. "Over-engineering" normally means making a system more complicated than it really needs to be to get the same job done. This usually tends to result in less robust systems, since more complex systems have more things that can go wrong.

On a different note from the other answers, there's been a few articles indicating that newer 'bone' like designs are being considered and implemented by companies like Airbus.

Airbus has partnered with Autodesk to rethink the design of those
  lowly partitions. Its new partition debuted today at the Autodesk
  University conference in Las Vegas and, thanks to 3-D printing and
  some wild new algorithms based on slime mold and bone growth, it
  weighs in at just 66 pounds. Airbus’s current partitions weigh 143
  pounds apiece. “Our goal was to reduce the weight by 30 percent, and
  we altogether achieved weight reduction by 55 percent,” says Bastian
  Schaefer, innovation manager at Airbus. “And we’re right at the
  beginning.”
  http://www.wired.com/2015/12/airbuss-newest-design-is-based-on-slime-mold-and-bones/

There are even some pretty impressive futuristic designs:

http://www.airbus.com/innovation/future-by-airbus/the-concept-plane/
http://www.smithsonianmag.com/arts-culture/aircraft-design-inspired-by-nature-and-enabled-by-tech-25222971/?no-ist

Rest assured, this concept plane pictured is as likely to become reality as those prognoses about the year 2000 from the Fifties. Writing this from my underwater habitat which can be reached by flying car only. Yeah, right!

@PeterKämpf sure which is why I lead with an article showing how these techniques are being considered (at small scale) today.  The point isn't that this is THE FUTURE but rather that there is room to improve current designs, in part using new and improved CAD techniques.  I figure the OP might be interested in such developments.

There are many small parts in today's aircraft which can benefit mightily from getting a little more attention. Most are left-over compromises which worked well enough not to be looked at in detail later, so they were never analyzed or optimized properly.

Is that structure really futuristic, or is it revisiting Barnes Wallis' WW2-era geodesics?

@PeterKämpf Care to elaborate on those details a bit? Sounds much more interesting than the picture above!

@PavelPetrman check this example out: http://www.wired.com/2015/12/airbuss-newest-design-is-based-on-slime-mold-and-bones/

I think calculating stresses and loads are no better now than 50 years ago. Engineers understood forces and physics no differently then than now. They just used a slide rule instead of software, but they got the same results. Instead of modeling things, they build real life ones and tested them. They got the same answers. They built a wing, bent it till it broke and discovered its strength and were able to build one to any load specification needed without overbuilding it. Modern software just gets results cheaper and faster.
The big differences between then and now are materials and system evolution. Not calculating forces and stresses.


text:
How would one define or explain the term "unloading the controls?"
I've heard people say "unload the controls" or "unload the wing/propeller."
What does this mean exactly? It is usually followed by releasing back pressure on the elevators or starting/ending a turn.

Starting a turn will increase load, not unload.  It's especially important in critical attitude dive recovery to level wings before pulling out.

Unload means immediately reduce the lift being produced by a lifting surface.  You:

unload a wing by pitching forward
unload a propeller by pitching forward, reducing power, or flattening blade angle (increasing RPM)

The typical reason to "unload" is to provide greater a margin from a stall or to reduce torque on an engine.
The term unload is easily understood from a bicyclist's perspective.  When a steep hill flattens or a bike is shifted into lower gear, the bicyclist experiences unloading.
In conventional aircraft, control force (called stick force gradient) exists to inhibit pilot over-control.  Control force is proportionate to the lift being produced and to aircraft speed.  Therefore, to say "unload the controls", is the same as saying to "unload the lift on that surface".

Great explanation.

When maneuvering the aircraft by deflecting the control surfaces with the stick, the wing with the downward aileron and the tail will generate additional lift, which increases the load on that wing (using the ailerons) and on the tailplane (using the elevator).
Unload the controls is a request to neutralize the pressure on the controls. Usually this means bringing the stick back to the center position, reducing the controls surface deflection, the amount of lift generated, and therefore reducing the aerodynamic load on the wings and tailplane.

When you are manipulating the airplane, for example in a turn, you're increasing the aerodynamic loading of the wings. In a 30-degree level turn, the loading is 1.3G, i.e. 1.3 times the weight of the aircraft in straight-and-level flight. In a 60 degrees level turn, the loading is 2G.
If the plane is flying with excessive aerodynamic loading for a continued amount of time, it can end up in a stall. This is because to achieve a higher loading, the angle of attack must be increased; eventually it may exceed the critical angle of attack. With a higher angle of attack, the airframe will also produce more drag, with slows the aircraft down.
Excessive loading can also lead to structural failure, as you are asking the wing / elevator structure to provide more lift, increasing the stress of the airframe.
"Unload the wings" is a general term meaning "to reduce the lift produced by the wings". You're asking too much from the airplane. The action to execute depends on the state of the aircraft. If you're in a level turn, reducing the bank angle reduces wing loading. If you're pulling up, easing back pressure on the yoke / stick reduces wing loading.
In aerial dogfighting, "unloading" means to ease back on the stick and allow the airplane to pick up speed. High-G loading bleeds off energy very quickly. A pilot must manage his energy during a fight. If he keeps making very sharp turns, the high aerodynamic loading of the airframe will slow the aircraft down; and slow moving targets are easy targets.

Excessive aerodynamic loading can lead to a stall. meaning the stall speed increases, right?

@Canuk excessive aerodynamic loading does not lead to a stall. F/A-18s have no problem making 9G turns. Increasing the angle of attack beyond the critical angle is the only thing that can lead to a stall (by definition).

you have it completely backwards. the AoA is defined as the angle between the chord of the wing and the relative airflow. As AoA increases, lift increases (until reaching the critical AoA). As lift increases, load increases. Not the other way around.

@rbp I meant to say "if you intentionally load up the wing for a continued amount of time, you might end up in a stall", not "aerodynamic stall is a result of high wing loading", though my wordings might get readers confused.

Maybe you want to go back and fix it. and delete the comments that are wrong. Also note that velocity is also a factor, and that intention has nothing to do with it.


text:
Lapsed Advanced Ground Instructor certificate, can I take a FIRC and and restore my privileges?
I hold an AGI certificate that I haven't exercised the privileges for in quite some time (more than 12 calendar months).
According to 14 CFR 61.217, can I simply take a Flight Instructor Refresher Course (online or in person) and restore my privileges?

§ 61.217 Recent experience requirements. The holder of a ground
  instructor certificate may not perform the duties of a ground
  instructor unless the person can show that one of the following
  occurred during the preceding 12 calendar months:
(a) Employment or activity as a ground instructor giving pilot, flight
  instructor, or ground instructor training;
(b) Employment or activity as a flight instructor giving pilot, flight
  instructor, or ground instructor ground or flight training;
(c) Completion of an approved flight instructor refresher course and
  receipt of a graduation certificate for that course; or
(d) An endorsement from an authorized instructor certifying that the
  person has demonstrated knowledge in the subject areas prescribed
  under § 61.213(a)(3) and (a)(4), as appropriate.

It appears so, but I guess at the root of my question is, is it really that simple or did I miss a regulation or sign-off requirement somewhere (I know I could get signed off in leiu of the FIRC).
Then to keep current, what constitutes activity as a ground instructor giving pilot...training?  Just teaching and logging that I've been teaching ground school, or would I actually have had to endorse some people to take the written exam during those 12 calendar months?

First, as the regulation says, you can indeed take an FIRC to restore your privileges.
Second, I couldn't find any FAA clarification on exactly what "employment or activity as a ground instructor" means, but the language is fairly clear: you just have to be able to show that you were providing instruction as a ground instructor. That could be via training records, invoices, employment contracts, written test endorsements etc.
As a general rule, there's no need to look for extra requirements that aren't stated in the regulations.

As a general rule, there's no need to look for extra requirements that aren't stated in the regulations. Hah! Yeah, that is for sure, its more that this one just seemed too straight-forward.


text:
What makes the ailerons on an aerobatic airplane different than the ailerons on a non-aerobatic airplane?
I often hear a spec of the "roll-rate" when talking about aerobatic airplanes and the term "fast ailerons." What exactly does that mean, and what makes the ailerons on an aerobatic airplane different than the ailerons on a non-aerobatic airplane?
How does the design of the ailerons affect the roll-rate of the airplane?

The term "fast ailerons" is an informal term that generally refers to the response of the plane relative to the pilot's input to the aileron controls on the stick or yolk. The faster the plane responds, the faster the ailerons (or other controls).
The roll rate is the speed at which a plane can complete a roll. This depends on a number of factors, including aileron size, maximum aileron deflection, wing length, and speed.
Ailerons that can deflect a lot of air without disrupting the airflow over the wing too much are desirable in an aerobatic plane, so the roll rate can be increased and so the plane will have better roll control at lower speeds. While larger ailerons are desirable on an aerobatic plane, they are also more susceptible to flutter at high speeds, which is not desirable because it tends to rip them off the wing. There are several design techniques that can be used to mitigate this, such as gap seals, counterweights, countersurfaces, etc.

One of the distinctive feature often found on aerobatics aircraft ailerons are spades. They are an efficient form of aerodynamic counterbalance that serve to reduce the effort needed on the stick to actuate the aileron.
They also serve to bring the centre of mass of the aileron forward of the hinge line in order to help prevent control surface flutter.

Pros

Being more efficient than "normal" ailerons (i.e. achieving higher roll rate), aeroboatics ailerons would require much more torque to actuate (without counterbalance).
The stick should be as "light" as possible to increase flight precision and reduce pilot fatigue .

Cons

Having a lighter control force can make it more difficult to find the centrepoint of the control quickly.

I asked Patty Wagstaff at Oshkosh one summer how she was a able to get the airplane to flip tail over nose - I couldn't envision a set of control movements that would make that happen.  She told me the plane was balanced on knife edge and that's what allowed it to be moved the way it was. The spades mentioned above help provide the light control forces to allow the control surfaces to make the large movements.


text:
What regulation or publication specifies that a TAF is good for a 5 statute mile radius around the reporting point?
It is commonly cited that a Terminal Aerodrome Forecast (TAF) is good for 5 statute miles around the airport it's forecasting for, but where is that 5 statute mile specification? I looked in the FAR/AIM and the and I could only find information about decoding them and when they were updated.
What publication specifies that the TAF covers a 5 statute mile radius (in the US)?

I've found it in two official US government documents where it describes the 5 SM radius for TAFs.
One from NOAA:

NWS TAFs consist of the expected meteorological conditions significant to aviation at an airport for a specified time period. For the U.S., this is the area within five (5) statute miles (SM) of the center of an airport’s runway complex.

Source: National Weather Service Instruction 10-813. November 18, 2020. Operations and Services. Aviation Weather Services, NWSPD 10-8. Terminal Aerodrome Forecasts. p. 5.
And one from the FAA in AC 00-45H - Aviation Weather Services - Change 2 on page 5-84 (emphasis added):

5.13 Terminal Aerodrome Forecast (TAF).
A TAF is a concise statement of the expected meteorological conditions significant to aviation for a specified time period within 5 sm of the center of the airport’s runway complex (terminal). The TAFs use the same weather codes found in METAR weather reports (see paragraph 3.1) and can be viewed on https://www.aviationweather.gov.

That is consistent with the definition of "vicinity" at ICAO level, in Annex 3, §4.4.2.7: "Vicinity— Between approximately 8 and 16 km of the aerodrome reference point and used only in METAR and SPECI with present weather in accordance with the template shown in Table A3-2". I guess 8 km comes from the conversion of 5 sm.

In the U.S., "vicinity" in the METAR or TAR is weather between 5 and 10 SM.

In weather reports (TAF and METAR) according to the AIM (https://www.faa.gov/air_traffic/publications/media/AIM.pdf) the abbreviation VC means Vicinity, but not at aerodrome. In the US METAR, 5 to 10 SM from the point of observation. In the US
TAF, 5 to 10 SM from the center of the runway complex. Elsewhere, within 8000m. (Fig 7-1-22) However, my question was, "where is it specified that TAFs cover 5 SM?" Which I found in the answer above. It makes sense then, that you'd assume the weather reported in the TAF was within that 5 SM radius, and if not, the TAF uses the qualifier VC.